Video hologram and device for reconstructing video holograms using geometrical calculation

ABSTRACT

A geometrical method for working out how to encode a hologram onto a hologram bearing medium is disclosed. The method involves (a) selecting a point on the three dimensional scene to be reconstructed; then (b) defining a virtual observer window through which the reconstructed three dimensional scene will be seen; then (c) tracing a pyramid from the edges of the observer window through the point and onto a region that forms only a portion of a hologram bearing medium; and finally (d) generating, solely in that region of the hologram-bearing medium, holographic information needed to generate a holographic reconstruction of the point.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/534,877 filed by A. Schwerdtner on May 12, 2005 entitled“Video Hologram and Device for Reconstructing Video Holograms”, thecontents of which are hereby incorporated by reference.

U.S. patent application Ser. No. 10/534,877 application is, in turn,related to, and claims priority from, PCT patent applicationPCT/DE03/03791 filed on Nov. 11, 2003 and to German Patent applicationDE 10253292.3 filed on Nov. 13, 2002, the contents of both of which arehereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a video hologram and a device forreconstructing video holograms having an optical system that includes atleast one light source, a lens and a hologram-bearing medium composed ofcells arranged in a matrix or an otherwise regular pattern, with atleast one opening per cell, and with the phase or amplitude of theopening being controllable.

BACKGROUND OF THE INVENTION

Devices for reconstructing video holograms using acousto-opticalmodulators (AOM) are known from prior art, as detailed in, for instance,U.S. Pat. No. 5,172,251 issued to Benton et al. on Dec. 15, 1992entitled “Three dimensional display system,” the contents of which arehereby incorporated by reference. Such acousto-optical modulatorstransform electric signals into optical wave fronts, which arerecomposed in a video frame using deflection mirrors to formtwo-dimensional holographic areas. A scene visible for the viewer isreconstructed from the individual wave fronts using further opticalelements. The optical means used, such as lenses and deflectionelements, have the dimensions of the reconstructed scenes. Due to theirgreat depth, these elements are voluminous and heavy. It is difficult tominiaturize them, so that their range of applications is limited.

Another way to generate large video holograms is the so-called “tilingmethod”, using computer-generated holograms (CGH). This method isdescribed in for instance PCT patent publication WO 00/75698 of PCTPatent application PCT/GB2000/001901 filed on May 18, 2000 by Pain et.al entitled “Holographic Displays” and in U.S. Pat. No. 6,437,919 B1issued to Brown, et al. on Aug. 20, 2002 “System for the production of adynamic image for display”, both of which are hereby incorporated byreference. In the tiling method, small CGHs having a small pitch arecreated using an optical system. In the first step of the method, therequired information is written to fast matrices that have a smallpitch, such as electronically addressable spatial light modulators(EASLM). These fast matrices are then reproduced on to a portion of asuitable holographic medium. A large video hologram is composed of thetiled replicas of the fast matrices. Usually, an optically addressablespatial light modulator (OASLM) is used as holographic medium. In asecond step, the composed video hologram is reconstructed with coherentlight in transmission or reflection.

In the CGH with controllable openings arranged in a matrix or in anotherwise regular pattern as described in, for instance, PCTpublications number WO 01/95016 A1 of PCT patent applicationPCT/GB2001/002302 filed on May 24, 2001 by Payne et al. entitled“Computation Time Reduction for Three-Dimensional Displays” or in Fukayaet al., “Eye-position tracking type electro-holographic display usingliquid crystal devices”, Proceedings of EOS Topical Meeting onDiffractive Optics, 1997, the diffraction on small openings is takenadvantage of for encoding the scenes. The wave fronts emerging from theopenings converge in object points of the three-dimensional scene beforethey reach the viewer. The smaller the pitch, and thus the smaller theopenings in the CGHs, the greater is the diffraction angle, i.e. theviewing angle. Consequently, with these known methods enlarging theviewing angle means to improve the resolution.

As is generally known, in Fourier holograms the scene is reconstructedas a direct or inverse Fourier transform of the hologram in a plane.This reconstruction is continued periodically at a periodicity interval,the extension of the periodicity interval being inversely proportionalto the pitch in the hologram.

If the dimension of the reconstruction of the Fourier hologram exceedsthe periodicity interval, adjacent diffraction orders will overlap. Asthe resolution is gradually decreased, i.e. as the pitch of the openingsincreases, the edges of the reconstruction will be increasinglydistorted by overlapping higher diffraction orders. The usable extent ofthe reconstruction is thus gradually limited.

If greater periodicity intervals and thus greater viewing angles are tobe achieved, the required pitch in the hologram comes closer to thewavelength of the light. Then, the CGHs must be sufficiently large inorder to be able to reconstruct large scenes. These two conditionsrequire a large CGH having a great number of openings. However, this iscurrently not feasible in the form of displays with controllableopenings, as discussed in, for instance, U.S. Pat. No. 6,831,678 issuedto Travis on Dec. 14, 2004 entitled “Autostereoscopic display”, thecontents of which are hereby incorporated by reference. CGH withcontrollable openings only measure one to several inches, with thepitches still being substantially greater than 1 μm.

The two parameters, pitch and hologram size, are characterized by theso-called space-bandwidth product (SBP) as the number of openings in thehologram. If the reconstruction of a CGH with controllable openings thathas a width of 50 cm is to be generated so that a viewer can see thescene at a distance of 1 m and in a 50-cm-wide horizontal viewingwindow, the SBP in horizontal direction is about 0.5×10⁶. Thiscorresponds to 500,000 openings at a distance of 1 μm in the CGH.Assuming an aspect ratio of 4:3, 375,000 openings are required in thevertical direction. Consequently, the CGH comprises 3.75×10¹¹ openings,if three color sub-pixels are taken into consideration. This number willbe tripled if the fact is taken into account that the CGH withcontrollable openings usually only allows the amplitudes to be affected.The phases are encoded taking advantage of the so-called detour phaseeffect, which requires at least three equidistant openings per samplingpoint. SLM having such a great number of controllable openings arehitherto unknown.

The hologram values must be calculated from the scenes to bereconstructed. Assuming a color depth of 1 Byte for each of the threeprimary colors and a frame rate of 50 Hz, a CGH requires an informationflow rate of 50*10¹²=0.5*10¹⁴ Byte/s. Fourier transformations of dataflows of this magnitude exceed the capabilities of today's computers byfar and do thus not allow holograms to be calculated based on localcomputers. However, transmitting such an amount of data through datanetworks is presently unfeasible for normal users.

In order to reduce the enormous number of computations it has beenproposed not to calculate the entire hologram, but only such parts of itthat can be seen directly by the viewer, or such parts that change. Thekind of hologram which consists of addressable sub-regions, such as theabove-mentioned “tiling hologram”, is disclosed in the above-mentionedpatent specification WO 01/95016 A1. Starting point of the calculationsis a so-called effective exit pupil, the position of which can coincidewith the eye pupil of the viewer. The image is tracked as the viewerposition changes by continuous recalculation of the hologram part thatgenerates the image for the new viewer position. However, this partlynullifies the reduction in the number of computations.

The disadvantages of the known methods can be summarized as follows:Arrangements with acousto-optical modulators are too voluminous andcannot be reduced to dimensions known from state-of-the-art flatdisplays; video holograms generated using the tiling method aretwo-stage processes which require enormous technical efforts and whichcannot easily be reduced to desktop dimensions; and arrangements basedon SLM with controllable openings are too small to be able toreconstruct large scenes. There are currently no large controllable SLMwith extremely small pitches, which would be needed for this, and thistechnology is further limited by the computer performance and datanetwork bandwidth available today.

SUMMARY OF THE INVENTION

The invention is defined in claim 1. In one implementation, videoholograms and devices for reconstructing video holograms withcontrollable openings according to the present invention arecharacterized in that in the viewing plane at least one viewing windowis formed in a periodicity interval as a direct or inverse Fouriertransform of the video hologram, said viewing window allowing a viewerto view a reconstruction of a three-dimensional scene. The maximalextent of the viewing window corresponds to the periodicity interval inthe plane of the inverse Fourier transformation in the image plane ofthe light source. A frustum stretches between the hologram and theviewing window. This frustum contains the entire three-dimensional sceneas a Fresnel transform of the video hologram.

The viewing window is limited approximately to and positioned inrelation to one eye, an eye distance of a viewer or to another suitablearea.

In an implementation, another viewing window is provided for the othereye of the viewer. This is achieved by the fact that the observed lightsource is displaced or added a second, real or virtual, adequatelycoherent light source at another suitable position to form a pair oflight sources in the optical system. This arrangement allows thethree-dimensional scene to be seen with both eyes through two associatedviewing windows. The content of the video hologram can be changed, i.e.re-encoded, according to the eye position in synchronism with theactivation of the second viewing window. If several viewers view thescene, more viewing windows can be generated by turning on additionallight sources.

In another implementation of the device for reconstructing a videohologram, the optical system and the hologram-bearing medium arearranged so that the higher diffraction orders of the video hologramhave a zero point for the first viewing window or an intensity minimumat the position of the second viewing window. This prevents the viewingwindow for one eye to cross-talk the other eye of the viewer or to otherviewers. It is thus taken advantage of the decrease in intensity of thelight towards higher diffraction orders, which is due to the finitewidth of the openings of the hologram-bearing medium and/or the minimaof the intensity distribution. The intensity distribution forrectangular openings, for example, is a sinc² function which rapidlydecreases in amplitude and forms a sin² function which decreases as thedistance grows.

The number of openings in the display determines the maximum number ofvalues that must be calculated for the video hologram. The transmissionof data from a computer or through a network to the display representingthe video hologram is limited to the same number of values. The dataflow rate does not substantially differ from the data flow rates knownfrom typical displays used today. Now, this will be illustrated with thehelp of an example.

If the viewing window is reduced, for example, from 50 cm (horizontal)by 37.5 cm (vertical) to 1 cm by 1 cm by choosing a sufficientlylow-resolution display, the number of openings in the hologram will dropto 1/1875. The required bandwidth is reduced in the same way during datatransmission through a network. Video holograms created with knownmethods require 10¹² openings, while this number is reduced to 510⁸pixels in this example. The scene can be viewed in full through theremaining viewing window. These requirements on pitch and hologram sizeaccording to the space-bandwidth product can already be fulfilled bydisplays available today. This allows the inexpensive realization oflarge real-time video holograms on displays with large pitches andhaving a large viewing window.

The viewing window is tracked by mechanically or electronicallydisplacing the light sources, by using movable mirrors or by using lightsources which can be adequately positioned in any other way. The viewingwindows are displaced according to the displacement of the light sourceimages. If the viewer moves, the light source(s) is (are) spatiallydisplaced so that the viewing windows follow the eyes of the viewer(s).This is to ensure that the viewers can also see the reconstructedthree-dimensional scene when they move, so that their freedom ofmovement is not limited. Several systems are known for detecting theposition of the viewers, e.g. systems based on magnetic sensors can beused beneficially for this.

An implementation of this invention also allows the efficientreconstruction of a video hologram in color. Here, the reconstruction isperformed with at least three openings per cell, representing the threeprimary colors. The amplitude or phase of the openings may becontrollable, and the openings may be encoded individually for each ofthe primary colors. Another possibility of reconstructing a videohologram in color is to perform at least three reconstructions one afteranother, namely for the individual primary colors, using the device ofthe present invention.

An implementation of this invention allows the efficient generation ofholographic reconstructions of spatially extended scenes throughcontrollable displays, such as TFT flat screens, in real-time andproviding large viewing angles. These video holograms can be usedbeneficially in TV, multimedia, game and design applications, in themedical and military sectors, and in many other areas of economy andsociety. The three-dimensional scenes can be generated by a computer orin any other way.

These and other features of the invention will be more fully understoodby references to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention is illustrated and explainedbelow in conjunction with the accompanying drawings, wherein

FIG. 1 is a general illustration of a video hologram and a device forreconstructing video holograms showing the generation of the diffractionorders and the position of a viewing window;

FIG. 1 a shows the content of FIG. 1 with a magnified view of an exampleof a focusing lens system 2 comprising two single lenses.

FIG. 2 is a general illustration of a device for reconstructing videoholograms showing a three-dimensional scene which can be viewed througha viewing window;

FIG. 3 is a general illustration of a device for reconstructing videoholograms showing the encoding of the three-dimensional scene in a partof the video hologram;

FIG. 4 is a diagram showing the light intensity distribution in theviewing plane depending on the diffraction orders; and

FIG. 5 is a general illustration of a device for reconstructing videoholograms showing the position of the viewing windows for both eyes of aviewer with regard to the diffraction orders to prevent cross-talking.

DETAILED DESCRIPTION

A device for reconstructing video holograms comprises thehologram-bearing medium, a sufficiently coherent, real or virtual, pointor line light source and an optical system. The video hologram-bearingmedium itself consists of cells which are arranged in a matrix or in anotherwise regular pattern with at least one opening per cell, the phaseor amplitude of said opening being controllable. The optical system forreconstructing the video hologram can be realized by an optical imagingsystem known in the art, consisting of a point or line laser or asufficiently coherent light source.

FIG. 1 shows the general arrangement of a video hologram and itsreconstruction. A light source 1, a focusing lens system 2, shown as asingle lens, for the purpose of simplicity, a hologram-bearing medium 3and a viewing plane 4 are arranged one after another, seen in thedirection of the propagating light. The viewing plane 4 corresponds withthe Fourier plane of the inverse transform of the video hologram withthe diffraction orders. FIG. 1A shows the content of FIG. 1 with amagnified view of an example of a focusing lens system 2 comprising twosingle lenses.

The light source 1 is imaged on to the viewing plane 4 through anoptical system, represented by the lens system 2. If a hologram-bearingmedium 3 is inserted, it (the hologram-bearing medium 3 being encodedwith a hologram) is reconstructed to comprise focal points (e.g., apoint 7 of a reconstructed three-dimensional scene 6 as shown in FIG. 3)before the viewing plane 4 (i.e., between the hologram-bearing medium 3and the viewing plane 4) and as an inverse Fourier transform in theviewing plane 4. The hologram-bearing medium 3 with periodic openingscreates equidistantly staggered diffraction orders in the viewing plane4, where the holographic encoding into higher diffraction orders takesplace, e.g. by way of the so-called detour phase effect. Because thelight intensity decreases towards higher diffraction orders, the 1^(st)or −1^(st) diffraction order is used as the viewing window 5. If notexplicitly expressed otherwise, the 1^(st) diffraction order will betaken as a basis in the further description of the invention.

The dimension of the reconstruction was chosen here to correspond withthe dimension of the periodicity interval of the 1^(st) diffractionorder in the viewing plane 4. Consequently, higher diffraction ordersare attached without forming a gap, but also without overlapping.

Being the Fourier transform, the selected 1^(st) diffraction order formsthe reconstruction of the hologram-bearing medium 3. However, it doesnot represent the actual three-dimensional scene 6. It is only used asthe viewing window 5 through which the three-dimensional scene 6 can beobserved (see FIG. 2). The actual three-dimensional scene 6 is indicatedin the form of a circle inside the bundle of rays of the 1^(st)diffraction order. The scene is thus located inside the reconstructionfrustum which stretches between the hologram-bearing medium 3 and theviewing window 5. The scene 6 is rendered as the Fresnel transform ofthe hologram-bearing medium 3, whereas the viewing window 5 is a part ofthe Fourier transform.

FIG. 3 shows the corresponding holographic encoding. Thethree-dimensional scene is composed of discrete points. A pyramid withthe viewing window 5 being the base and the selected point 7 in thescene 6 being the peak, is prolonged through this point and projected onto the hologram-bearing medium 3. A projection area 8 is created in thehologram-bearing medium 3 that point being holographically encoded inthe projection area. The distances between the point 7 to the cells ofthe hologram-bearing medium 3 can be determined in order to calculatethe phase values. This reconstruction allows the size of the viewingwindow 5 to be constrained by the periodicity interval. If, however, thepoint 7 was encoded in the entire hologram-bearing medium 3, thereconstruction would extend beyond the periodicity interval. The viewingzones from adjacent diffraction orders would overlap, which would resultin the viewer seeing a periodic continuation of the point 7. Thecontours of a surface encoded in this manner would appear blurred due tomultiple overlapping.

The intensity decrease towards higher diffraction orders is takenadvantage of to suppress cross-talking to other viewing windows. FIG. 4shows schematically a light intensity distribution over the diffractionorders, said distribution being determined by the width of the openingsin the CGH. The abscissa shows the diffraction orders. The 1^(st)diffraction order represents the viewing window 5 for the left eye, i.e.the left viewing window, through which the three-dimensional scene canbe viewed. Cross-talking into a viewing window for the right eye issuppressed by the decrease in light intensity towards higher diffractionorders and, additionally, by the zero point of the intensitydistribution.

Of course, the viewer can view the scene 6 of the hologram 3 with botheyes (see FIG. 5). For the right eye, the right viewing window 5′represented by the −1^(st) diffraction order of the light source 1′ waschosen. As can be seen in the drawing, this light influences the lefteye at a very low intensity. Here, it corresponds to the −6^(th)diffraction order.

For the left eye, the 1^(st) diffraction order corresponding to theposition of the light source 1 was chosen. The left viewing window 5 isformed likewise. According to an implementation of this invention, thecorresponding three-dimensional scenes 6 and 6′ (not shown) arereconstructed using the light sources 1 and 1′ in a fix position inrelation to the eyes. For this, the hologram 3 will be re-encoded whenthe light sources 1 and 1′ are turned on. Alternatively, the two lightsources, 1 and 1′, can simultaneously reconstruct the hologram 3 in thetwo viewing windows 5 and 5′.

If the viewer moves, the light sources 1 and 1′ are tracked so that thetwo viewing windows 5 and 5′ remain localized on the eyes of the viewer.The same applies for movements in the normal direction, i.e.perpendicular to the video hologram.

Further, several viewers can view a three-dimensional scene ifadditional viewing windows are created by turning on additional lightsources.

Although the invention has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the invention defined in the appended claims is not necessarilylimited to the specific features or acts described. Rather, the specificfeatures and acts are disclosed as exemplary forms of implementing theclaimed invention. Modifications may readily be devised by thoseordinarily skilled in the art without departing from the spirit or scopeof the present invention.

1. A method of generating a sequence of holograms to be encoded into ahologram-bearing medium of a device comprising an optical system, atleast one light source and the hologram-bearing medium having a matrixof cells, the optical system imaging the light source into an imageplane of the light source, the method comprising the steps of: (a)selecting a point in the three dimensional scene to be reconstructed;(b) defining an observer window through which the reconstructed threedimensional scene will be seen; the maximal extent of the observerwindow corresponding to a periodicity interval in the plane of theinverse Fourier transformation of the hologram-bearing medium in theimage plane of the light source; (c) tracing a pyramid from the edges ofthe observer window through the point and onto a region that forms onlya portion of a hologram bearing medium; (d) encoding, solely in thatregion of the hologram-bearing medium, a hologram needed to generate aholographic reconstruction of the point, the region being variable inposition and size, the position and the size of the region depending onthe location of the point and on the location of the observer window. 2.The method of claim 1 in which the reconstructed three dimensional scenecomprises multiple such points.
 3. The method of claim 1 in which thehologram is illuminated by a light source and an optical system suchthat only when an observer's eye is positioned approximately at theimage plane of the light source can the holographic reconstruction beseen properly.
 4. The method of claim 1 in which the reconstructed pointis visible from a defined viewing position, and is characterized inthat: the region (a) encodes information for that reconstructed pointand (b) is the only region in the hologram-bearing medium encoded withinformation for that point, and (c) is restricted in size to form aportion of the entire hologram-bearing medium, the size being such thatmultiple reconstructions of that point caused by higher diffractionorders are not visible at the defined viewing position.
 5. The method ofclaim 1 comprising the step of generating the hologram by timesequentially re-encoding a hologram on the hologram-bearing medium forthe left and then the right eye of an observer.
 6. The method of claim 1in which the holographic reconstruction of the point is the Fresneltransform of the hologram and not the inverse Fourier transform of thehologram.
 7. The method of claim 1 in which the encoding is such that,on reconstruction, an inverse Fourier transform of the hologram isgenerated at a viewing plane at which the observer's eyes must beplaced.
 8. The method of claim 1 in which the three dimensional scene isreconstructable anywhere within a volume defined by the hologram bearingmedium and the observer window.
 9. The method of claim 1 in which theobserver window is smaller than the hologram-bearing medium.
 10. Themethod of claim 1 in which there are separate observer windows, one foreach eye of an observer.
 11. The method of claim 10 in which eachobserver window is approximately 1 cm×1 cm.
 12. The method of claim 1 inwhich the location of an observer's eyes are tracked and the position ofthe observer windows is altered so that the observer can maintain a viewthrough each observer window even when moving his or her head.
 13. Themethod of claim 1 in which the size of the observer window is calculatedas a function of the periodicity interval of the hologram bearingmedium.
 14. The method of claim 1 in which the hologram bearing mediumis a TFT flat screen.
 15. The method of claim 1 in which thehologram-bearing medium is a display screen in a television.
 16. Themethod of claim 1 in which the hologram-bearing medium is a displayscreen in a multimedia device.
 17. The method of claim 1 in which thehologram-bearing medium is a display screen in a gaming device.
 18. Themethod of claim 1 in which the hologram-bearing medium is a displayscreen in a medical image display device.
 19. The method of claim 1 inwhich the hologram-bearing medium is a display screen in a militaryinformation display device.
 20. A holographic reconstruction generatedfrom a hologram encoded using the method defined in claim
 1. 21. Acomputer adapted to generate a sequence of holograms to be encoded intoa hologram-bearing medium of a display device using the method definedin claim
 1. 22. A method of generating a holographic reconstruction of athree dimensional scene using a display device and a computer, thedevice including a light source and an optical system to illuminate ahologram-bearing medium; comprising the steps of: (a) using the computerto encode a hologram on the hologram-bearing medium; the hologram havingbeen generated using the method of claim 1; (b) illuminating thehologram bearing medium using the light source and optical system sothat the reconstructed three dimensional scene is visible.
 23. Thesequence of holograms generated by the method according to claim
 1. 24.A display device for generating a holographic reconstruction of a threedimensional scene, the display device comprising an optical system, thatat least one light source and a hologram-bearing medium, the opticalsystem imaging the light source into an image plane of the light source,the hologram-bearing medium being encoded with a sequence of holograms,the sequence of holograms being generated by using the method defined inclaim 1.